This invention relates to a plate having at least one friction surface for a multiple-plate clutch, in particular for a wet-running switchable multiple-plate clutch. Multiple-plate clutches on which the present invention is based are known in general in a variety of modifications. For example, reference may be made to the variants of the embodiments described in German Patent 28 54 051 A1, U.S. Pat. No. 4,280,609 or German Patent 35 32 759 C1.
Accordingly, the essential components of such multiple-plate clutches to which reference is made below include two or more plates having an essentially ring-shaped shape. If the multiple-plate clutch includes a plurality of plates, as is the case in the majority of such multiple plate clutches, then a first group of plates, the so-called outer plates, are arranged on an outer plate carrier, called the drive ring, which is connected to a shaft, e.g., a drive shaft, and a second group of plates, the so-called inner plates, are arranged on an inner plate carrier which is connected to a shaft, e.g., an output shaft. The plates of the two groups engage in the manner of gears to form a plate package, with one outer plate being adjacent to an inner plate. Outer plates and inner plates are displaceable relative to one another in the axial direction and can be brought into or out of frictional engagement with their adjacent end faces in pairs, i.e., transmitting torque from the drive shaft to the output shaft.
The plates on which the present invention is based are known from the state of the art in a plurality of modifications. For example, there are known plates which have a friction lining, which is usually made of a fibrous material or the like, on one or both essentially ring-shaped end faces of a preferably metallic carrier. There are also those made of a single material in one piece, in particular those made of a friction material of the aforementioned type. Reference is gain made to the discussions and citations in the aforementioned publications as examples.
As a rule, at least one of the two end faces—the so-called friction surfaces—which can be brought into frictional connection with one another, of adjacent plates is provided with a specially designed surface structuring and/or with a pattern grooves.
For example, International Patent WO 97/32678 describes surface structuring of so-called steel plates, where this structuring causes an increase in the coefficient of friction of the steel and friction coating surfaces of adjacent plates which are in contact with one another.
On the other hand, the clutch plate illustrated in FIG. 2 of U.S. Pat. No. 4,280,609 as well as the clutch plate illustrated in FIG. 4 of German Patent 28 54 051 A1 both have a so-called waffle pattern of grooves on their friction surfaces, where a plurality of parallel linear grooves are intersected by a plurality of parallel linear grooves running at right angles to the former. The reason for such a groove structure lies in the necessity of cooling the surfaces which rub one against one another, this cooling being implemented by a stream of coolant passing through these grooves.
Although plates have been developed for a wide variety of equipment transmitting a torque, e.g., clutches, brakes and the like, and the plates have been adapted to the different stresses accordingly, there are still unwanted problems for many applications. Namely, there is still certain frictional contact between adjacent plates for a variety of reasons even when the equipment is not being operated, i.e., when no torque is to be transmitted but there is, nevertheless, a torque, the so-called drag torque, which is transmitted from the drive shaft to the output shaft or vice-versa.